1. Field of Invention
This invention is directed to systems and methods for detecting streaks, such as streaks caused by imperfections appearing in the field of view of, or defects in, scanning systems that employ linear imaging arrays.
2. Description of Related Art
A variety of systems and methods are conventionally used in digital scanning. Typical scanning systems employ imaging arrays that are stationary with respect to the devices in which they are housed, or which are caused to move relative to a stationary substrate to be scanned. In such systems, constant or variable rate movement is effected between the image to be scanned and the imaging array. The imaging array takes a rapidly sequential series of pictures of the image medium that is presented to be scanned as the relative movement is effected. The scanning system then digitizes the image scanned for storage and/or reproduction.
Typical imaging arrays include multiple rows of individually pixilated sensors, each discrete pixel sensor being disposed to take these rapidly sequential digital images of the column of the image which is presented within that pixel sensor's field of view.
A difficulty in such systems is that any imperfection in the field of view of the imaging array either over a single sensor or over a group of sensors is repeatedly scanned and digitized as a dark spot in the scanned image. Alternatively, a defective sensor at a given pixel sensor location, in its failure to be able to scan, can likewise potentially be digitized as a dark spot. The result is that the reproduced output image from the device can include a streak or line corresponding with the position of the imperfection or the defective sensor as the image of the dark spot is repeatedly reproduced to form the streak or line.
Various exemplary scanning devices employ stationary Full-Width Imager Arrays (FWA) or linear Charge-Coupled Device (CCD) arrays. In such systems in which the imaging arrays are stationary, the arrays are often separated from the image medium to be scanned by a platen glass, or other transparent surface or lens, overlying the imaging array. Various exemplary methods are employed in these devices to move the image medium to be scanned across the transparent surface. One such exemplary method, commonly referred to as Constant Velocity Transport (CVT) scanning, employs a belt, drum or other like device to move the image medium to be scanned across a linear sensor, such as, for example, an imaging array of individually pixilated sensors.
A recognized problem in imaging array scanning systems, such as, for example, CVT scanning devices, occurs when dirt particles, stray paper fibers, contaminants, imperfections, obstructions and/or other objects appear on the transparent surface, or otherwise in the field of view of the imaging array. Such dirt particle, paper fiber, contaminant, imperfection, obstruction and/or other object remaining stationary on the transparent surface, or otherwise in the field of view over the imaging array, is repeatedly imaged and digitized as though it were part of the scanned image. The result is that the image reproduced by the exemplary CVT scanning device will include a streak or a line on the subsequently reproduced image.
A variety of systems and methods are employed to ensure that a platen glass or other overlying transparent surface is kept free of dirt particles, paper fibers, contaminants, imperfections, obstructions and/or other objects. These systems and methods are directed to improving reproduced image quality. Often the methods are manual, requiring visual inspection and manual cleaning of an exemplary system's platen glass or transparent surface. These methods can be mechanically automated in that a wiper blade, or the like, may be employed to routinely, automatically “wipe down” the platen glass or transparent surface. While limitedly effective in removing surface contaminants from a transparent surface, none of these methods account for obstruction or imperfection actually on an individual sensor, or for the potential of a defective sensor, providing the same sort of false scanned image data as an obstruction in the field of view of the array would produce on the overlying transparent surface.
Other systems and methods for improving reproduced output image quality include automated system steps that review the reproduced output image in comparison with the scanned image in order to detect, and optionally compensate for, differences before the reproduced image is ultimately displayed. One complication with such an approach is that it is difficult to determine whether a line which is digitized and reproduced as part of the output image is actually part of the image that was scanned or whether it was produced because the imaging array repeatedly scanned an imperfection stationary in its field of view, or otherwise input false or non-image data from one or a series of defective sensors, and then reproduced the non-image data as a line or streak in the reproduced image.
Difficulties in detecting dirt particles, paper fibers, contaminants, imperfections, obstructions and/or other objects are particularly acute as elements of foreign matter which commonly produce streaks in conventional and exemplary CVT scanning systems are extremely small. Any dirt particle, paper fiber, contaminant, imperfection, obstruction and/or other object generally large enough to be detected by simple visual inspection is, most often, large enough to be picked up by the scanned image medium as the scanned image medium is moved across the platen glass or other overlying transparent surface. The dirt particle, paper fiber, contaminant, imperfection, obstruction and/or other object is moved out of the way, swept away with the movement of the image medium being scanned. Dirt particles, paper fibers, contaminants, imperfections, obstructions and/or other objects that are very small, on the other hand, have a greater tendency to adhere to the transparent surface and to resist being swept away by the movement of the scanned image medium.
Among other systems and methods employed to remove streaks from digital images produced in CVT scanning systems, software algorithms exist that review stored image data representing the scanned image in its entirety. Such software algorithms apply image data revisions before reproducing a digitally scanned image in an output display device. A disadvantage in such methods is that full scanned image review algorithms require the scanning of the entire image and storage of the data representing the entire scanned image before reviewing the data and applying necessary image data revisions. In high speed scanning applications, this requirement to record an entire image and then review and revise that image before reproducing the image has the potential to slow the scanning and reproduction process to an unacceptable level.
U.S. patent application Ser. No. 10/740,493, filed Dec. 22, 2003, which is incorporated herein by reference in its entirety, discloses various exemplary embodiments for detecting streaks, such as streaks caused by imperfections in scanning systems that employ linear imaging arrays where the scanned image is sampled a minimum of two times by separate positionally discrete rows or sets of sensors in the imaging array. The at least two samples, though very close together, correspond to separate discrete linear scan locations. The scanned image data obtained from each of the two discrete rows is then normalized to adjust for different filtering capabilities, particularly color filtering in multi-spectral imaging arrays, in the discrete rows of the imaging array. The normalized scanned input image data from at least two discrete rows is then compared on a one for one basis in an attempt to detect imperfections overlying one or more rows of sensors in the imaging array.